A comparative immunocytochemical and immunochemical analysis of glycoproteins synthesized in the bovine subcommissural organ

Calcitonin-like immunoreactivity in the subcommissural organ-Reissner's fiber complex of some freshwater and marine teleosts

The subcommissural organ (SCO) is a highly specialised circumventricular ependymal organ covering and penetrating the posterior commissure. The secretory products of the SCO condense to form Reissner's fiber (RF). Because of its extensive secretory activity and the chemical properties of its secretion, the organ functions as similar to the neurosecretory cells. Teleosts comprised of more than 20,000 extant species that show great diversity in terms of the form, habit and habitat. Affinity of calcitonin antibodies for the SCO-RF complex was used as a histochemical tool to study the morphology of some freshwater and seawater teleosts and its potential correlate to their osmotic environment. While intense to moderate calcitonin-like immunoreactivity was seen in the cells of the SCO of majority of the freshwater species viz., common carp, catfish, eel and perch; the SCO of goldfish revealed limited immunoreactivity. Like the SCO, the RF in all species was also immunostained with antibodies against calcitonin. It appeared as a single, continuous fiber that ran from SCO into the third ventricle and extended through the aqueduct, fourth ventricle and central canal of the spinal cord. In contrast to that in the freshwater fishes, the SCO-RF complex in majority of the seawater fishes, showed no calcitonin-like immunoreactivity. The data presented in this study described the comparative histomorphology of the SCO-RF complex and suggest a possibility that the calcitonin-like immunoreactivity in the SCO-RF complex might be a feature correlated to the osmotic environment of the fish.

Multiple forms of glycoproteins in the secretory product of the bovine subcommissural organ--an ancient glial structure

The glial subcommissural organ (SCO) is a conserved structure of the vertebrate brain that secretes a glycoprotein-rich product into both the extracellular matrix and the cerebrospinal fluid of the third ventricle that forms Reissner's fibre (RF). In order to identify specific secretory proteins of the subcommissural organ, a panel of antigen- and epitope-specific monoclonal antibodies was raised against bovine RF to study the distribution of epitopes in Western blots of bovine RF. Six groups of epitopes that were specific for SCO secretion were distinguished on the basis of their phylogenetic conservation and their different grades of resistance against chemical denaturation. The monoclonal antibody aRFME 4 recognised a carbohydrate-containing epitope that was strongly conserved in vertebrates and unique for SCO secretion. All epitopes showed essentially the same distribution pattern over 15 bovine RF glycoprotein fractions of different molecular masses in immunoblots indicating that the different RF fractions are closely related. They may represent multiple forms of SCO spondin.

Immunological cross-reactivities between proteins secreted by the subcommissural organ, and plant lectins

The glial subcommissural organ (SCO) discharges a glycoprotein-rich secretory product into the third ventricle to form Reissner's fibre (RF). The SCO proteins bear N-linked oligosaccharides, such as mediate many cell-cell and cell-matrix interactions. In such interactions the corresponding partner molecule recognising the sugar chain is often a sugar-specific protein (lectin). We present here evidence that the constituents of the SCO secretory product include proteins immunologically cross-reactive with certain plant lectins. Polyclonal antisera directed against Phaseolus vulgaris agglutinin-L (PHA-L) labelled the apically released RF-material of the rat SCO. Indirect ELISA studies shows in addition that anti-RF antisera bound to certain plant lectins (PHA-L, Con A). Dot spot assays demonstrated binding of anti-PHA-L to RF proteins. In Western blots of RF proteins anti-PHA-L, anti-RCA and anti-Con A bound to distinctive subsets of the RF protein fractions.

The bovine subcommissural organ: cytochemical and immunochemical characterization of the secretory process

Specific glycoproteins of the bovine subcommissural organ (SCO) were studied by means of various techniques: light and electron microscopy, immunoaffinity chromatography, electrophoresis and Western blotting. Use of lectins (Con A, WGA, PHA-E and -L, LCA) allowed to specify the synthesis and release of complex-type glycoproteins that bear high-mannose-carbohydrate chains in their precursor forms and probably triantennary carbohydrate chains in their mature forms. Antibodies raised against SCO extracts were characterized by means of various tests and used to purify specific compounds. Immunopurified fractions using A99 polyclonal antibody contained numerous polypeptides reactive with Con A, their apparent molecular weight (MW) ranging from 240 to 50 kDa. Only two glycopeptides were strongly labeled with WGA (98 and 52/54 kDa MW). Immunopurified fractions using C1B8A8 monoclonal antibody, specific of the complex-type glycoproteins at different steps of glycosylation, showed three specific Con A-reactive polypeptides at 88, 54 and 34 kDa MW. Only the 34 kDa glycopeptide was strongly labeled with WGA. The latter could correspond to the monomeric form of the secreted compound. Electrophoretical analyses of Reissner's fiber material allowed the detection of a WGA-positive smear in the upper part of the blots, suggesting that the complex-type glycoproteins, when released into the CSF, constitute a stable polymer.

The complex-type glycoprotein secreted by the bovine subcommissural organ: an immunological study using C1B8A8 monoclonal antibody

The secretory pathway of the complex-type glycoprotein specific to the subcommissural organ (SCO) was examined using the monoclonal antibody (Mab) C1B8A8. Immunoreactive material was revealed in various compartments of the secretory ependymocyte, i.e., the endoplasmic reticulum, the Golgi area and the secretory vacuoles. In addition, immunoreactive material was also observed in the ventricular cavity. Evidence of a release both at the apical lining and at the basal process of the SCO ependymocytes suggests that the same protein could be secreted into the cerebrospinal fluid and the perivascular spaces. After immunoaffinity chromatography of soluble extracts of the SCO on Mab C1B8A8 immunoadsorbent columns, three glycopeptides were identified on Western blots; they were concanavalin A (Con A)-positive (88, 54 and 34 kDa) and wheat-germ agglutinin (WGA)-positive (54 and 34 kDa). The Con A-positive glycopeptide (88 kDa) is probably related to the high-mannose-type glycoprotein, the precursor form of the secreted compound, whereas the 54 kDa-glycopeptide that is both Con A- and WGA-positive could represent an intermediate form. The 34 kDa-glycopeptide that is strongly WGA-positive could be related to the monomeric form of the secreted compound. These three glycopeptides were not revealed in eluted fractions of soluble extracts of the ependyma that served as control.